A navigation system performs travel guidance for enabling a user to easily and quickly reach the selected destination. A typical example is a vehicle navigation system where a user drives a car having a vehicle navigation system. Such a navigation system detects the position of the vehicle, reads out map data pertaining to an area at the current vehicle position from a data storage medium, for example, a CD-ROM (compact disc read-only memory), a DVD (digital versatile disc), or a hard disc. Alternatively, such map data can be provided to the vehicle from a remote server through a communication network such as Internet. The navigation system displays a map image on a monitor screen while superimposing a mark representing the current location of the user's vehicle on the map image.
When a destination is set, the navigation system starts a route guidance function for setting a guided route from the start point to the destination. To determine the guided route to the destination, the navigation system calculates and determines an optimum route to the destination based on various parameters. For example, the guided route is determined based on the shortest way to reach the destination, the route preferring freeways to surface roads, the least expensive way to the destination, or the route without using toll road, and the like.
Typically, the route guidance function performs an intersection guidance process in which a monitor screen displays an enlarged intersection diagram and the direction in which the vehicle is to travel while displaying the guided route on a map. During the route guidance, the navigation system reads the nodes data from the data storage medium such as DVD and successively stores the nodes data of road segments (expressed in longitude and latitude) constituting the guided route in a memory.
During actual traveling, the node series stored in the memory is searched for a portion of the guided route to be displayed in a map display area of the monitor screen, and the portion of the guided route is highlighted so as to be discriminable from other routes. When the vehicle is within a predetermined distance of an intersection it is approaching, an intersection guidance diagram (an enlarged or highlighted intersection diagram with an arrow indicating the direction in which the vehicle is to turn at the intersection) is displayed to inform a user of the desired one of roads or directions selectable at the intersection. Such route guidance by the navigation system is also given by voice instruction.
As in the foregoing, the navigation system is able to guide the user to the destination through the calculated route. However, when the vehicle travels in an off-road area, such as mountains or desert, etc., where no road segments data is provided in the map data storage of the navigation system, it is not possible for the navigation system to guide the user. Moreover, when the vehicle is away from the roads, and if the user does not remember the trail of the travel to the current position, he may not be able to go back to the roads.
U.S. Pat. No. 6,356,837 (the '837 patent) provides a navigation system which is able to record the trail of the off-road travel in a memory. Typically, such a trail of the off-road travel is displayed on a map image on a screen of the navigation system based on the data recorded in the memory. The navigation system disclosed in the '837 patent includes an automatic mode and a manual mode for recording the off-road trail. In the automatic mode, the navigation system automatically starts recording the trail when the vehicle position is away from the roads (off-road). In the manual mode, a user controls ON/OFF of the off-road trail of the navigation system.
The automatic mode prevents the user from inadvertently forgetting to start the recording when the vehicle leaves the road. However, this mode would exhaust the available memory space for trail recording more quickly compared to the driver's judicious use of the manual mode recording. This is because the automatic trail mode will keep recording the vehicle movements even when the user does not need any recording, such as driving in a large parking lot of a neighborhood shopping mall (where the map database does not have any streets).
Also, in the automatic mode, if the vehicle runs on a rural highway in an area where the map database is not yet available, the trail memory can be exhausted in a short time. Increasing the size of trail memory would raise the hardware cost, and may introduce another problem. Deleting unwanted trail recordings would become time consuming because the user would have to review an excessive number of trails before deletion.
In the manual mode, in the case where the user forgets to turn on the off-road travel recording until he has already been driving far away from the roads, it is not possible to record the off-road trail from the roads to the current position. Therefore, there arises a problem that not only the off-road trail data is incomplete, but also the user may not be able to return to the original road. Further, in the '837 patent, when the trail memory has not a sufficient capacity such as when the memory is full or almost full, it is not possible to record the off-road trail.